In the related art, there exist heat pipes which transport heat by the latent heat of a fluid (working fluid or operating fluid). Among these heat pipes, a loop heat pipe has an evaporator and a condenser that are connected to each other by a vapor pipe and a liquid pipe so as to form a loop. The evaporator causes the working liquid to evaporate when heated from the outside. The condenser causes vapor to condense by dissipating heat to the outside. Such configuration of a loop heat pipe (also called circulating loop heat pipe) is discussed in Japanese Patent No. 4,459,783 with reference to FIG. 1 thereof.
A loop heat pipe is a device that efficiently transports heat by utilizing latent heat produced by evaporation/condensation of a working fluid. A characteristic feature of a loop heat pipe is that the pressure difference between a working fluid in liquid phase and a working fluid in gaseous phase (vapor), and the capillary force of a wick works as the driving force, and thus no external electric power is used to transport heat. A loop heat pipe generally includes an evaporator with a wick built in to vaporize a working fluid, a compensation chamber that temporarily stores the working fluid, a condenser that turns the vapor of the working fluid into a liquid, and a transport pipe that connects the evaporator and the condenser to each other. Depending on the state of the working fluid flowing inside the transport pipe, the portion of the transport pipe which conveys a liquid from the condenser to the evaporator is called a liquid pipe, and the portion of the transport pipe that conveys a vapor from the evaporator to the condenser is called a vapor pipe.
A loop heat pipe with a plurality of evaporators built in a single evaporator section may transport heat by dispersing the amount of input heat among each of the evaporators, thereby enabling cooling of a high-heat generating element. Such a loop heat pipe is used to cool an electronic apparatus such as a computer. For example, this loop heat pipe is attached to an electronic component that is a high-heat generating element such as an integrated circuit mounted on a circuit board built in a computer, and cools the electronic component. A loop heat pipe with a plurality of wicks built in the evaporator is illustrated as FIGS. 1A and 1B.
A loop heat pipe 30 illustrated as FIGS. 1A and 1B has a plurality of wicks 5 (three wicks in this example) built in an evaporator 1. For example, the wicks 5 are made of a porous material using ceramic or nickel, or metal such as copper, copper oxide, or stainless steel as a raw material, or a porous material using a high polymer material such as polyethylene resin as a raw material. The evaporator 1 is provided with a liquid-side manifold 11 and a vapor-side manifold 12. The liquid-side manifold 11 supplies a working fluid 6 that has been returned from a compensation chamber 8 to each of the wicks 5. The vapor-side manifold 12 causes a vapor 7 generated from each of the wicks 5 to flow into a vapor pipe 3. At the interface between each of the wicks 5 and the case of the evaporator 1, heat propagates to the surface of the wick 5 from the case, causing the working fluid 6 that has seeped into the surface of the wick 5 to evaporate and turn into the vapor 7.
As illustrated as FIGS. 1C and 1D, the wick 5 has a cylindrical shape, and has a hollow 5H that opens at the liquid pipe 4 side. The hollow 5H defines a liquid channel that facilitates supply of the working fluid 6 to the outer periphery of the wick 5. On the outer periphery of the wick 5, a plurality of grooves 5G extend from the liquid pipe 4 side to the vapor pipe 3 side so that the vapor that has evaporated quickly moves to the vapor pipe 3. Each of the grooves 5G defines a vapor channel. The inside of the loop heat pipe 30 is completely evacuated first, and then filled with a liquid such as ammonia or a water-based, an alcohol based, hydrocarbon compound-based, or fluorine hydrocarbon compound-based liquid as the working fluid 6. In the wick 5 of the evaporator 1 applied with heat, the working fluid 6 in liquid phase turns into the vapor 7, and then flows through the vapor pipe 3. In the condenser 2, the vapor 7 turns into the working fluid 6 in liquid phase, and then returns to the evaporator 1. The capillary pressure of the wick 5 is used as a pumping pressure to cause the working fluid 6 to circulate between the evaporator 1 and the condenser 2.
FIG. 2 illustrates, in exploded view, the configuration of the evaporator 1 illustrated as FIGS. 1A and 1B. A wick accommodating section 1W is provided in the portion of the case between the liquid-side manifold 11 and the vapor-side manifold 12. Each of the wicks 5 is accommodated in the wick accommodating section 1W, forming the evaporator 1. A bottom 1B of the evaporator 1 is attached over an integrated circuit 22, which is a heat-generating circuit component mounted on a circuit board 21, via a heat spreader 23.
While the evaporator 1 configured as mentioned above may be manufactured by vertically segmenting the evaporator 1 as illustrated as FIG. 2, the evaporator 1 may be also manufactured by segmenting the evaporator 1 along the direction of flow of the working fluid 6 as illustrated as FIG. 3. In the manufacturing method illustrated as FIG. 3, the liquid-side manifold 11 that connects to the liquid pipe 4, the wick accommodating section 1W that accommodates the wick 5, and the vapor-side manifold 12 that connects to the vapor pipe 3 are first manufactured separately, and then joined together. The method of manufacturing the evaporator 1 by segmenting the evaporator 1 along the flow of the working fluid 6 as illustrated as FIG. 3 may achieve higher efficiency of heat transfer to the wick 5, because a gap is less likely to develop between the wick 5 and the wick accommodating section 1W.
FIG. 4A illustrates the operations of the evaporator 1 and compensation chamber 8 when heat input to the evaporator 1 illustrated as FIGS. 1A, 2, and 3 from the integrated circuit 22 is uniform. FIG. 4B illustrates a local cross-section taken along the line IVB-IVB in FIG. 4A. Now, let T1 be the temperature on the integrated circuit 22 side of the evaporator 1, and let T2 be the temperature on the side opposite to the integrated circuit 22 (Let T2a, T2b, and T2c be the temperatures in corresponding areas of the evaporator 1 above the three wicks 5). In a case where there is uniform heat input from the integrated circuit 22, and the heat inputted to the evaporator 1 is dispersed among each of the wicks 5, the temperatures T2a, T2b, and T2c in the areas of the evaporator 1 above the corresponding wicks 5 are substantially equal. In this case, the difference in the amount of vapor generated in each of the wicks 5 is small, and also, the working fluid 6 that has been returned is supplied in accordance with the amount of vapor. Thus, the evaporator 1 operates properly.
An example of a loop heat pipe is disclosed in, for example, JP 4459783, in which FIG. 1 illustrates an overall configuration of the loop heat pipe.